The present invention relates to an electric discharge machining apparatus for machining a workpiece with the discharge energy of an electric discharge produced between an electrode and the workpiece.
Electric discharge machining are classified into apparatus in which a workpiece is machined by a rod-shaped electrode and apparatus in which a workpiece is machined by relatively moving the workpiece and a wire electrode passing through a hole which has been defined as by drilling in the workpiece.
One conventional electric discharge machining apparatus using a wire electrode will be described with reference to FIG. 1 of the accompanying drawings. As shown in FIG. 1, a workpiece 1 has a hole 1a defined therethrough, and a wire electrode 2 extends through the hole 1a, with an insulating liquid 3 supplied between the workpiece 1 and the wire electrode 2. The insulating liquid 3, hereinafter referred to as a machining liquid, is delivered from a tank 4 by a pump 5 and ejected by a nozzle 6 into a gap (interelectrode gap) between the workpiece 1 and the wire electrode 2.
The workpiece 1 and the wire electrode 2 are relatively moved by moving a table 11 on which the workpiece 1 is placed. The table 11 is moved by a Y-axis motor 13 and an X-axis motor 12. The relative movement of the workpiece 1 and the wire electrode 2 occurs in a two-dimensional plane lying on the X- and Y-axes.
The wire electrode 2 is supplied from a wire supply reel 7 and guided by lower and upper wire guides 8A, 8B to pass through the workpiece 1. The wire electrode 2 then travels through an electric energy feeder 9 and is wound on a wire take-up and tension roller 10.
The X- and Y-axis motors 12, 13 are driven and controlled by a control unit 14 which may comprise a numerical control unit (NC unit), a profiling unit, or a controller employing a computer.
The electric energy is generated by a machining power supply unit 15 comprising, for example, a DC power supply 15a, a switching device 15b, a current-limiting resistor 15c, and a control circuit 15d for controlling the switching device 15b.
Operation of the conventional EDM apparatus shown in FIG. 1 is as follows: A high-frequency pulse voltage is applied by the machining power supply unit 15 between the workpiece 1 and the wire electrode 2. Part of the workpiece 1 is melted and scattered by a discharge explosion produced by one pulse. During this time, the interelectrode gap is filled with a gas and is ionized. Therefore, a certain quiescent time is required before a next pulse voltage is applied. If the quiescent time were too short, an electric discharge would be produced again in the same position before the gap is sufficiently insulated, causing wire electrode 2 to be melted.
Therefore, where an ordinary machining power supply unit is employed, it is customary to machine the workpiece under such electric conditions as not to cause the wire to be cut off, for example, by controlling the quiescent time of the power supply unit 15 dependent on the type and thickness of the workpiece, etc. Thus, the machining speed is considerably lower than a theoretically possible limit speed. Furthermore, the wire electrode 2 will be melted away if it is not uniform but varies in thickness or it has a protuberance, a flaw, or other defect, causing discharge concentration.
To prevent the wire electrode 2 from being broken in the conventional wire cutting electric discharge machining apparatus, it has been the prior practice to reduce the output energy from the machining power supply unit 15, for example, to avoid wire breakage even when the discharge is concentrated on a certain point on the wire electrode 2. This however has resulted in a very low machining speed.
One conventional solution has been to ascertain whether the machining condition is good or bad or to check the electrode for its condition immediately prior to damage, and to take a safety measure by returning the machining process automatically back to a normal machining condition or avoiding damage to the electrode, based on the result of the checking process, so that the machining speed will be prevented from being lowered.
The most general way of ascertaining whether the machining condition is good or bad or checking the electrode for its condition immediately prior to damage is to observe the average value of the voltage applied between the electrode and the workpiece. When the average voltage is low, the interelectrode impedance is low, indicating that the insulation for electric discharges is liable to be lost and a discharge concentration (which is most responsible for wire breakage) is occurring due to short-circuiting or a deposit of sludge or machining chips.
When machining with a small interelectrode gap (which is indispensable for high-accuracy machining), however, frequent short circuits take place even when the interelectrode gap is in a normal condition. The machining efficiency is lowered if the short circuits are detected for taking a safety measure.